The invention is based on a level measuring device for a fuel tank of a motor vehicle.
A level measuring device of this kind is known from DE 39 14 637 A1. This level measuring device has a measuring tube disposed in the fuel tank, which extends from the bottom of the fuel tank to at least the height of the maximal fill level of the fuel tank and into which the fuel is delivered. The level measuring device also has a differential pressure sensor, whose one measurement input is connected to the lower end of the measuring tube and whose other measurement input is subjected to the fluid pressure at the bottom of the fuel tank. The measuring tube has an overflow opening which is disposed at least at the height of the maximal fill level of the fuel tank. The measuring tube can be emptied by means of an opening disposed close to the bottom of the fuel tank or by means of a valve at the height of the current level of the fuel tank. The level determination takes place in an evaluation device based on the difference of the output signals of the differential pressure sensor when the measuring tube is filled up to the overflow opening and when the measuring tube is emptied to the current level of the fuel tank. The delivery of fuel into the measuring tube takes place by way of a return line, through which fuel not consumed by the internal combustion engine of the motor vehicle is delivered back into the fuel tank. The fuel flowing through the return line is possibly heated so that there is a temperature difference between the fuel in the measuring tube and the fuel in the fuel tank, which in turn results in a difference in the density of the fuel in the measuring tube in comparison to the density of the fuel in the fuel tank, which leads to errors in the output signals of the differential pressure sensor. Furthermore, fuel delivery systems are also known in which there is no return from the engine to the fuel tank so that the known level measuring device cannot be used.